D-region rocket measurements in winter anomaly absorption conditions

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THE winter anomaly in ionospheric absorption was first identified by Appleton1, and although extensively studied over the past 40 yr it still remains for the most part a major unsolved problem. Its principal feature is that at latitudes above ∼35°, on most winter days, h.f. ionospheric absorption is large (often larger than on summer days at 1200). Furthermore, there is considerably greater variability in the day-to-day absorption in winter than in summer, and the magnitude of the anomaly seems to increase with latitude, although at high latitudes the phenomenon is largely masked by other disturbances characteristic of the auroral zone. Rocket measurements have shown that on winter anomaly days there is a considerable enhancement of the electron density of the lower ionosphere particularly in the height range 80–90 km (refs 2–4). Calculations indicate that these enhanced electron densities would be sufficient to account for the winter anomaly absorption without involving any changes in other parameters which determine the magnitude of h.f. absorption such as the collisional frequency v.